139La NMR evidence for phase solitons in the ground state of overdoped manganites

TL;DR

This study provides experimental evidence using 139La NMR that the ground state of overdoped La(1-x)Ca(x)MnO(3) manganites is characterized by inhomogeneous solitonic charge and spin modulations, confirming theoretical predictions.

Contribution

The paper presents the first direct experimental confirmation of orbital and charge solitons in overdoped manganites, advancing understanding of their ground state properties.

Findings

NMR spectra show wipeout in AFM phase at low T

Broad NMR spectrum reappears below 30K indicating incommensurate modulation

Emergence of narrow feature at very low T suggests solitonic ground state

Abstract

Hole doped transition metal oxides are famous due to their extraordinary charge transport properties, such as high temperature superconductivity (cuprates) and colossal magnetoresistance (manganites). Astonishing, the mother system of these compounds is a Mott insulator, whereas important role in the establishment of the metallic or superconducting state is played by the way that holes are self-organized with doping. Experiments have shown that by adding holes the insulating phase breaks into antiferromagnetic (AFM) regions, which are separated by hole rich clumps (stripes) with a rapid change of the phase of the background spins and orbitals. However, recent experiments in overdoped manganites of the La(1-x)Ca(x)MnO(3) (LCMO) family have shown that instead of charge stripes, charge in these systems is organized in a uniform charge density wave (CDW). Besides, recent theoretical works predicted that the ground state is inhomogeneously modulated by orbital and charge solitons, i.e. narrow regions carrying charge (+/-)e/2, where the orbital arrangement varies very rapidly. So far, this has been only a theoretical prediction. Here, by using 139La Nuclear Magnetic Resonance (NMR) we provide direct evidence that the ground state of overdoped LCMO is indeed solitonic. By lowering temperature the narrow NMR spectra observed in the AFM phase are shown to wipe out, while for T<30K a very broad spectrum reappears, characteristic of an incommensurate (IC) charge and spin modulation. Remarkably, by further decreasing temperature, a relatively narrow feature emerges from the broad IC NMR signal, manifesting the formation of a solitonic modulation as T->0.